Ciliophora

(invertebrate zoology) The ciliated protozoans, a homogeneous subphylum of the Protozoa distinguished principally by a mouth, ciliation, and infraciliature.

A subphylum of the Protozoa. The ciliates are a fairly homogeneous group of highly differentiated, unicellular organisms. Over 5000 species have been described, and many more surely exist but remain to be discovered. Typically, ciliates are larger than most other protozoans, ranging from 10 to 3000 micrometers (about 1/2500 to 1/8 in.). Some larger species are easily visible to the naked eye. The majority of them are free-living forms, found abundantly in a variety of fresh- and salt-water habitats, although a few entire groups live in association with other organisms, generally as harmless ecto- or endocommensals. Their principal value to humans is as experimental animals in a host of biological investigations.

A classification scheme for the ciliates is given below. It is widely recognized as the most reasonable and useful classification currently available. Separate articles appear on each group listed. Subordinal divisions of some orders are commonly recognized by protozoologists.

Subphylum Ciliophora

     Class Ciliatea

          Subclass Holotrichia

                         Order: Gymnostomatida

                                                       Trichostomatida

                                                       Chonotrichida

                                                       Apostomatida

                                                       Astomatida

                                                       Hymenostomatida

                                                       Thigmotrichida

          Subclass Peritrichia

                              Order Peritrichida

          Subclass Suctoria

                              Order Suctorida

          Subclass Spirotrichia

                         Order: Heterotrichida

                                                       Oligotrichida

                                                       Tintinnida

                                                       Entodiniomorphida

                                                       Odontostomatida

                                                       Hypotrichida

The usual ciliate life cycle is fairly simple. An individual feeds and undergoes binary fission, and the resulting filial products repeat the process. Some commensal or parasitic forms have a more complicated life history. Some ciliates, including free-living species, have a cystic stage in their cycle. As in other kinds of Protozoa this stage often serves as a protective phase during adverse environmental conditions, such as desiccation or lack of food. It also may be important in distribution, and thus possibly in preservation, of the species.

Six major characteristics aid in distinguishing the Ciliophora from other protozoan groups. Not all of these are entirely unique, but when taken together they are definitely distinctive of ciliates: mouth, ciliation, infraciliature, nuclear apparatus, fission, and reproduction.

Most Ciliophora possess a true mouth or cytostome often associated with a buccal cavity containing compound ciliary organelles. However, some ciliates are completely astomatous, that is, mouthless. Nutrition is heterotrophic in ciliates.

The Ciliophora possess simple cilia or compound ciliary organelles, often in abundance, in at least one stage of their life cycle. Morphologically, cilia are relatively short and slender hairlike structures, whose ultrastructure is known, from electron microscope studies, to be composed of nine peripheral and two central fibrils. Membranes and membranelles are characteristically associated with the mouth or buccal areas and serve to bring food into the oral opening, although they sometimes aid in locomotion as well. See also Cilia and flagella.

Infraciliature is present, without exception, at a subpellicular level in the cortex. The infraciliature consists essentially of basal bodies, or kinetosomes, associated with cilia and ciliary organelles at their bases, plus certain more or less interconnecting fibrils.

Ciliophora possess two kinds of nuclei, and at least one of each is usually present. The smaller, or micronucleus, contains recognizable chromosomes and behaves much as the single nucleus in cells of metazoan organisms. The larger, or macronucleus, is considered indispensable in controlling metabolic functions, and is recognized as having genic control over all phenotypic characteristics of ciliates.

Ciliophora exhibit a type of binary fission commonly known as transverse division. In ciliates the splitting results in two filial organisms, the anterior or proter and the posterior or opisthe which, geometrically speaking, show homothety with respect to identical structures possessed by each. Thus, homothetogenic is both a broader and most exact descriptive term.

Ciliophora lack true sexual reproduction. Ciliates do not show syngamy, with fusion of free gametes. Processes such as conjugation are considered to be sexual phenomena, since meiosis and chromosome recombination are involved, but not sexual reproduction. In addition to conjugation, certain ciliates exhibit forms of sexual phenomena known as autogamy and cytogamy. See also Protozoa; Reproduction (animal).